1. Field of the Invention
The invention relates to a method for operation of several adjacent magnetic-inductive flow meters, each of the flow meters comprising a measurement tube through which an electrically conductive medium has flowed, a magnetic field generating apparatus for permeating the medium with a magnetic field which comprises a component perpendicular to the longitudinal axis of the measurement tube, and a measurement apparatus for measuring the voltage which has been induced into the medium and for determining the flow rate from the induced voltage. Furthermore, the invention also relates to an arrangement of several adjacent magnetic-inductive flow meters, each of the adjacent flow meters having a control apparatus and the control apparatus implementing the aforementioned method.
2. Description of Related Art
Magnetic-inductive flow meters have been widely known in the prior art for decades. Reference is made by way of example to the literature citation Technical Flow Rate Measurement by Dr. Eng. K. W. Bonfig, 3rd edition, Vulkan-Verlag Essen, 2002, pp. 123 to 167 and moreover to the literature citation Principles of Magnetic-Inductive Flow Rate Measurement by Cert. Eng. Friedrich Hoffmann, 3rd ed., publication of the company KROHNE Messtechnik GmbH & Co. KG, 2003.
The basic principle of a magnetic-inductive flow meter for measuring the flow rate of a flowing medium goes back to Michael Faraday who suggested, as early as 1832, the use of the principle of electromagnetic induction for measuring the flow velocities of an electrically conductive medium. According to the Faraday Law, in such a flowing medium which is permeated by a magnetic field, an electrical field intensity arises perpendicular to the flow direction of the medium and perpendicular to the magnetic field. The Faraday Induction Law is used in magnetic-inductive flow meters in that, by means of a magnetic field generating apparatus which conventionally has two magnetic field coils, a magnetic field is generated which changes over time during a measurement process and the magnetic field at least partially permeates the electrically conductive medium which is flowing through a measurement tube. The generated magnetic field has a component perpendicular to the flow direction of the medium and the part of the measurement tube touching the medium is electrically insulating. The electrical field intensity produced by induction in the medium can be measured, for example, by electrodes which are electrically in contact with the medium as electrical voltage or are capacitively detected by electrodes which are not electrically in contact with the medium. Then, the flow rate of the medium through the measurement tube is derived from the measured signals. The measurement error from the magnetic-inductive flow meters known from the prior art is less than 0.2%.
For the magnetic-inductive flow meters underlying the invention as prior art reference is made, by way of example, to German Patent Application Nos. 197 08 857, 10 2004 063 617 (which corresponds to U.S. Pat. No. 7,261,001), 10 2008 057 755 (which corresponds to U.S. Patent Application Publication 2010/0126282) and 10 2008 057 756 (which corresponds to U.S. Patent Application Publication 2010/0132478). The disclosure content of these documents is hereby expressly incorporated by reference in this patent application.
In a host of applications, it is necessary to arrange and operate several magnetic-inductive flow meters adjacent to one another. A first and a second magnetic-inductive flow meter are adjacent for the following considerations if at least the magnetic field produced by the magnetic field generating apparatus of the first flow meter at least partially permeates the measurement tube of the second flow meter. Of course, an adjacent arrangement is not limited to two flow meters. Often, it is not possible, for example, under limited space conditions, to choose the spatial distance from the magnetic-inductive flow meters to be so great that they are not adjacent. Shielding of the flow meters would be associated with additional costs and effort.
If the first flow meter and the second flow meter in operation carry out measurement processes, it is unknown, on the one hand, whether the measurement processes of the two adjacent flow meters overlap in time, and on the other hand, how great the time overlap, which is generally not constant, is in the case of a time overlap.
If a time overlap of the measurement processes of the two adjacent flow meters is assumed, in the measurement tube of the second flow meter, the magnetic field which has been produced by the magnetic field generating apparatus of the second flow meter and the magnetic field which has been generated by the magnetic field generation apparatus of the first flow meter and which extends as far as the measurement tube of the second flow meter are superimposed. The superposition of the two magnetic fields results in an induced electrical voltage which varies in an unknown manner and a corresponding influence on the flow rate measurements; this means a reduction of the measurement accuracy. Thus, for example, at a constant flow rate through the measurement tube of one flow meter, a varying flow rate can be indicated by the flow meter. Of course, the measurement process of the second flow meter also influences the measured value of the flow rate of the first flow meter.